The present invention relates to an image sensor which reads picture-information and electrically transmits it, and which is suitable for a facsimile or image scanner apparatus.
In a prior art linear image sensor, the light-receiving element of the linear image sensor is typically as disclosed in Japanese Patent Disclosure S61-124171. The structure is shown in FIG. 5. In FIG. 5, symbol 4 is a separated (or separation) layer, symbol 3 is an epitaxial layer and collector, symbol 6 is a base, and symbol 9 is an emitter.
However, since the junction capacitance is large and the base region is in a floating state because the junction portion between the base and collector is wide, there has been a problem of an after-image in which the last signal of a bright state remains in the readout signal even when the read out image changes from a bright state to a dark state so that a distinct picture is not obtained.
To solve the above problem, the sensor of the present invention comprises a light-receiving MOS diode in which a part of a light-receiving element is covered with a light-transmissive electrode for transmitting a part of light, and letting the light-receiving MOS diode operate in an inversion state while accumulating photo-charge, so that the generated photo-charge is accumulated in the inversion region thereof and base region of phototransistor. By placing the light-receiving MOS diode in an accumulating state during a reset time after readout to the outside and resetting through the emitter in a state in which photo-charge density of the base region is raised to move residual charge without performing readout to the base region, residual charge causing an after-image is reduced. By placing the light-receiving MOS diode in the accumulating state even during readout lets photo-charge accumulated under the light-receiving MOS diode move to the base region, and enlarging the voltage between the base and emitter makes bipolar transistor operation of the phototransistor easy. This characteristic is improved even when the read picture changes from a dark state to a bright state or, conversely from the above-mentioned operation, from a bright state to a dark state, in which rising of a readout signal is a little lower than rising of a signal of enough bright state.
By forming the inversion region of the light-receiving diode and the base of the phototransistor with the same conductivity type, elements constructing a unit light-receiving element are reduced. In order to transfer photo-charge accumulated at the inversion region under the light-receiving MOS diode to the base of the phototransistor uniformly and surely between a plurality of light-receiving elements, the gate electrode of the light-receiving MOS diode covers at least a part of the base of the phototransistor through isolating coating. By forming a MOS transistor for switching to read a signal out from the light-receiving elements outside on a substrate of conductive (or conductivity) type which is different from the conductive type of the substrate that the light-receiving MOS diode is formed with and by making the gate electrodes common, element area is reduced.
By forming the base of the phototransistor with a well of a C-MOS transistor of a peripheral circuit, a base process added to C-MOS manufacturing process is reduced so as to realize a low cost linear image sensor.
By not applying constantly electric field of more than 0.7 MV/cm to the gate oxide film of the light-receiving MOS diode, influence of surface recombination current having impact is stable. Further by reducing threshold voltage of the light-receiving MOS diode, the diode operates keeping inversion state even at readout, and surface recombination current of oxide film boundary which is active by depletion becomes small.